Supermassive Black Hole Binaries in the Multi-messenger Context of Ground and Spaceborne VLBI

Abstract

Evidence of a gravitational wave background suggests the existence of a population of sub-parsec supermassive black hole binaries (SMBHBs), with characteristic angular separations on the order of 1–10 μas. Spaceborne extensions of Very Long Baseline Interferometry (VLBI) and the next-generation ground arrays introduce the possibility of directly imaging these SMBHBs. In this work, a binary spectral energy distribution model is used to predict the detectability of SMBHBs with ground and spaceborne VLBI. We consider the Black Hole Explorer (BHEX), a proposed spaceborne VLBI mission, as our primary case study. We explore the detectable SMBHB parameter space and identify distinguishable binary signatures in the visibility domain. We find that for a flux-density-limited sample, ground array observations are more effective at detecting a wider region of the binary parameter space, with M_tot ≳ 10⁹ M_⊙ systems detectable out to redshift z = 0.075 and beyond, depending on mass ratio. Conversely, inclusion of a spaceborne element such as BHEX, offering finer angular resolution (∼6 μas) and sampling of the (u, v) plane not limited by Earth rotation synthesis, will provide significant benefits in constraining binary properties.An illustrative Fisher analysis shows improvements in characterisation of the separation and position angle of SMBHBs by a factor of ∼4. Near-future ground and/or spaceborne VLBI may achieve the first direct observation of a SMBHB, contributing significantly to multi-messenger studies of such systems with pulsar timing arrays and observations across the electromagnetic spectrum.